1. Field of the Invention
The present invention relates to a transformer to be used in an inverter system high-frequency heating device and a transformer unit having the transformer.
2. Description of the Related Art
The inverter system high-frequency heating device includes a transformer unit 73 mounting a transformer 72 on a printed board 71 as shown in FIG. 17.
A circuit for the transformer unit 73 will be described with reference to FIG. 18.
A commercial power source 74 is full-wave rectified by a rectifying circuit 75 such as a diode bridge and the power is converted into a high-frequency voltage by an inverter 76, and the high-frequency voltage is applied to a primary winding 77 of the transformer 72. Consequently, a high voltage having a high frequency of several kV is generated on a secondary winding 78 of the transformer 72. The high voltage having the high frequency is rectified by a voltage doubler rectifying circuit 81 comprising a capacitor 79 and a diode 80. Thus, the high voltage is applied to a magnetron 82 to be a microwave generator. Moreover, a heater winding 83 of the transformer 72 is connected to a filament 84 of the magnetron 82 to heat the filament 84. The magnetron 82 oscillates a microwave by heating the filament 84 and applying the high voltage.
As shown in FIG. 19, the transformer 72 to be used in the transformer unit 73 has such a structure that a bobbin 85 has the primary winding 77, the secondary winding 78 and the heater winding 83 wound concentrically and a core 86 is inserted from both sides into the center of the bobbin 85. As shown in FIG. 20, the transformer 72 has a primary connecting pin 87, a secondary connecting pin 88, a heater connecting pin 89 and a ground connecting pin 90 in a bottom portion thereof, and is inserted in the through hole of the printed board 71 to be a mounting source and is connected to the circuit pattern of the printed board 71 by soldering.
FIG. 21 shows an example of the circuit pattern of the printed board 71, in which through holes 87A, 88A, 89A and 90A for inserting the primary connecting pin 87, the secondary connecting pin 88, the heater connecting pin 89 and the ground connecting pin 90 in the transformer 72 therethrough are formed. The through holes 88A and 89A inserting the secondary connecting pin 88 and the heater connecting pin 89 of the transformer 72 therethrough are formed in a high-voltage component region A, respectively. In the high-voltage component region A, moreover, a part of the transformer 72 is mounted, and furthermore, high-voltage components constituting the voltage doubler rectifying circuit 81, for example, the capacitor 79 and the diode 80 are mounted. In a region other than the high-voltage component region A of the printed board 71, moreover, various components constituting a weak voltage circuit such as a control circuit are mounted.
The transformer having the structure shown in FIG. 19 has been described in JP-A-2001-189221 gazette (FIG. 3), for example.
The high-frequency heating device has required a reduction in a size and an enhancement in a function. Therefore, there has been used a component having the size of each portion reduced and a higher added value.
However, the high-voltage components such as the capacitor 79 and the diode 80 which constitute the voltage doubler rectifying circuit 81 are much larger than components constituting the control circuit, and the size of the high-voltage component region A of the printed board 71 is increased in order to maintain a space for mounting these high-voltage components.
In addition, in the transformer unit 73, it is necessary to increase an interval between circuit patterns in order to prevent a leakage between the primary side to be the input side of the transformer 72 and the secondary side having a high voltage to be the output side. Consequently, the size of the high-voltage component region A in the printed board 71 is to be increased.
In the transformer unit 73 having the above structure, thus, the size of the high-voltage component region A is to be increased. Therefore, the printed board 71 itself becomes large-sized so that it is hard to meet the requirement for reducing the size of the high-frequency heating device.
On the other hand, as shown in FIG. 22, the high-voltage components such as the capacitor 79 and the diode 80 which constitute the voltage doubler rectifying circuit 81 are mounted on another small board 91, and the small board 91 is also erected on the printed board 71. With such a structure, however, the small board 91 is required separately. Consequently, a cost is increased. In addition, a large installation space in the printed board 71 is occupied by the small board 91. Thus, the size cannot be reduced.